In mass and/or rapid transit operation, the railway vehicles, primarily passenger cars, are generally equipped with disc brake units. Each of the disc brake units includes a hydropneumatic pressure operated actuator portion and a caliper assembly. The actuator portion of each disc brake unit functions to accept brake cylinder air and to amplify the pressure eleven (11) to twelve (12) times to a hydraulic pressure. The hydraulic pressure then acts on a piston to provide the force to tongs of the caliper assembly. That is, the caliper assembly and the actuator portion work together during a brake application to move the tongs of the caliper and position the brake heads with the attached brake shoes so that a gripping force is applied to the braking surface of an axle mounted disc. Generally, an automatic slack adjuster is built into the actuator to maintain the proper clearance between the brake shoes and the disc brake unit as the brake shoes wear. The caliper assembly normally consists of a swivel bracket, two tongs with attached brake heads, and a bifurcated support bracket. The tongs are pivotally mounted to the swivel bracket, and the swivel bracket is pivotally connected to the bifurcated support bracket. The bifurcated support bracket is used to mount the disc brake unit to the railway vehicle and, in some cases, to provide an attachment for the parking brake portion. In practice, the tongs transmit the output force of the actuator through the fulcrum to the brake heads to clamp the brake shoes or pads onto the disc brake unit. The swivel bracket is attached to two depending legs of the bifurcated support bracket by a swivel pin which provides a relative movement to the disc brake unit. This flexibility is necessary because the bifurcated support bracket, in some cases, is attached to the sprung portion of the vehicle to which the disc brake unit is directly attached to the wheels or axle of the vehicle.
Recently, a railway transit operator reported a breakage failure of the caliper braking unit in which the long leg of the bifurcated support bracket broke at a point where it joined the bracket. Upon examination of the braking unit, the shorter leg of the bifurcated support bracket remained in tact, but appeared to be slightly bent outwardly as a result of the failure. Upon closer examination, it was found that the swivel pin was firmly seized in the longer broken leg, and that the end of the swivel pin associated with the shorter leg was severely pitted and corroded. The corroded end was out of tolerance and was undersized by several hundredths of an inch, and the associated bushing was heavily corroded and oversized by several hundredth inches. Upon further examination, it was observed that the center or intermediate portion of the swivel pin was relatively clean, and its diameter was within the manufacturing tolerances, as was the inside diameter of the bushing located in the swivel bracket. The frozen end of the swivel pin, lodged in the bushing of the longer leg, was difficult to move, and required a considerable amount of force to separate. However, the outer diameter of the center portion of the swivel pin and the inner diameter of the associated bushing were within the manufacturing tolerances. It was concluded from the way that the swivel pin and bushings wore, that the broken leg was the result of overloading which occurred during braking operations. It will be appreciated that when the wheel turns in a counterclockwise direction, as viewed from the outside of the railway vehicle, a clockwise moment is exerted on the swivel bracket via the brake head and the brake tong. This tends to twist the swivel bracket, which is disposed between the two legs of the bifurcated support bracket. Thus, the swivel bracket bears against the inside edge near the end of the longer leg, and also against the inside edge near the top of the shorter leg. In normal operation, the longer leg slightly bends outwardly and reaches a point where the load is transmitted by the swivel pin to the shorter leg so that both legs share the load equally. However, if the end of the swivel pin and/or the associated bushing in the shorter leg are severely worn or corroded, the longer leg is permitted to bend much further before the swivel pin transmits any load to the shorter leg. At some point, the bending force exceeds the tensile stress of the ductile iron support bracket so that the longer leg fails by being broken.